Channel assignment selection reducing call blocking and call cutoff in a cellular communication system

ABSTRACT

Cellular communication systems supporting high utilization geographic regions having extensive cell overlap segments that collectively contain a substantial portion of the mobile units. A system and method for channel assignments incorporating selection from alternative transceivers defining overlapping cells is provided with load balancing to reduce call blocking. The system incorporates selective multiple handoffs responsive to channel assignment requests both to extend load balancing and also to substantially avoid call cutoff when active mobile units cross cell boundaries into possibly saturated cells.

This is a Continuation of application Ser. No. 08/442,336, filed on May16, 1995 now U.S. Pat. No. 5,633,915.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to cellular communication systems, such ascellular telephone or personal communication services (PCS), and moreparticularly relates (a) to a multilayer cellular design in whichmultiple cellular arrangements (each with an assigned group offrequencies) provides a substantial degree of coverage overlap, and (b)to a method for allocating and transferring calls among the cellulararrangements.

2. Discussion of the Background

Conventional cellular systems have a hierarchical system design. Amobile switching office is attached by voice and data links to a numberof base stations, each of which is connected to an antenna with a set offrequencies, each of which can connect to a number of mobile units(HHTs) via a radio channel in its predetermined portion of regioncoverage. An HHT can be a hand-held telephone or other mobile unitcommunicating voice or data over an assigned frequency channel to aselected base station. Throughout this specification, when voicecommunication is discussed, the communication channel created and thecommunication links could be purely data, voice or hybrid voice and datacommunication.

The mobile switching office and base stations have the computing powerto process communicating an HHT's requests for service and to determinewhich frequency channel assignment will be initially allocated forcommunicating with the HHT, as well as any hand-off reassignment ofchannel and antenna necessitated by the HHT moving beyond the cell ofthe currently assigned antenna.

A common approach to cellular design is illustrated in FIG. 1 andincludes a hexagonal lattice of cells with a single antenna coveringeach cell. The actual portion of the region covered by an antenna may beslightly larger than the hexagonal cell, as shown by the circular regionof radius R in FIG. 1. The overlap of cells at the cell boundariesidentifies the cell segments in which conventional systems may hand-offthe channel assignment and antenna for an HHT moving across a callboundary. However, this cellular overlap covers only a small portion ofthe geographical area of a cell.

When an HHT with an assigned channel moves to a new cell where theantenna covering the cell has an available channel, the hand-offchanging the antenna and frequency for both transmit and receive istransparent to the user. If the antenna in the new cell has no availablechannel, the call in progress is cut off, this being an unfortunateproblem with current cellular systems.

The frequencies used for channel assignments are limited. In a cellularsystems the frequency set allocated to a given cell may be reused atsome specified distance such as the distance D shown in FIG. 1. Thisdistance must be large enough so as to not create co-channelinterference with HHTs using the same channel in different cells. Thedistance D in FIG. 1 allows the bold-faced seven-cell cluster to berepeated to cover an arbitrarily large geographical region with allfrequencies reused repetitively at the same distance D.

The literature teaches various systems (see, for example, FIG. 4 of "TheCellular Concept," V. M. MacDonald, Bell Systems Technical Journal, Vol.58, No. 1, Pages 15-41, January, 1979, incorporated by reference herein)of patterns of clusters of distinct frequency sets which may be reusedat a certain safe distance from each other.

In cellular systems with such frequency reuse allowing coverage ofarbitrarily large regions, there is still a problem in that the numberof telephone calls that may be active in a given cell at any moment arelimited by the number of frequencies allocated to that cell. Somedigital systems have improved the total number of calls possible in eachcell by multiplexing calls and employing more complex HHTs. Still, thenumber of active calls in any given cell is limited. When this number isreached by active calls in a cell and not near the boundary where theycould be handed off, any new HHT in that cell requesting service will beblocked. Note that blocking can occur even though neighboring cells haveavailable channels.

One solution to the problem of excessive call blocking and call cut-offsis to reduce the cell size, providing a multitude of low powermicrocells which increases the total available channels over ageographical region by increasing frequency reusage. However, the powerof a microcell cannot be reduced too low or the reliability ofcommunication will suffer. Moreover, decreases in transceiver powercause the background noise to signal strength ratio to grow requiringgreater HHT complexity to reject the noise levels incurred. Furthermore,as the call size decreases, moving HHTs will require more hand-offs,increasing system overhead and the chance that moving active HHTS willbe cut off. This increases the risk of the entire system or portionsthereof going into a "thrashing" situation. During a "thrashing"situation, cut-offs of existing calls become a real risk. The cut-off ofan active telephone call is considered more disruptive than theunavailability of a channel to a new request for service. Thus, themobility of HHTs over the region and the level of background noise serveto yield a practical limit to the minimum cell size that may be providedover a region. When the cell size is as small as practical the inabilityto operate using a majority of channel capacity without noticeable callblockage is a problem with current systems.

Another solution is to have different sets of frequencies occur withdifferent reuse distances, yielding layers of various size cells, wherethe smaller size cells possessing increased frequency reuse may serveonly a non-contiguous portion of the region supplementing the contiguouscell region of another layer. Such multiple reuse patterns addcomplexity to the system with the smaller cell portion still susceptibleto more background noise and greater need for hand-offs. Furthermore,the number of assignable channels in multiple frequency reuse distancesystems may vary so as to provide considerably less capacity in portionsof each original cell causing surges in those areas to be moredisruptive.

Another problem with current systems is that the boundary area betweencells is a portion of the region where relatively small movement of anHHT can necessitate a hand-off, and oscillatory movement of an HHTacross a boundary or circular motion around the intersection point wherethree adjacent hexagons meet can greatly increase the occurrence ofhand-off overheads while at best preserving a low grade of signalstrength to an HHT at such a local in the region. There is a need incellular systems to avoid the disruptive behavior of service to HHTshappening at the cell boundaries.

Another problem that exists in conventional systems is that a failure, arepair, or the line of a given antenna, which takes the cell off theair, will result in a dead area of coverage in which no availableservice can occur in that cell for some period of time, and if an HHTmoves into that cell while communication is in progress, thecommunication will be cut off.

The current cellular system has two service providers, and the directionof PCS service, particularly in metropolitan areas, is to have two ormore providers offer competing cellular service over the same broadregion. The partition of available channels to a multitude of providers,each operating independently and each subject to the degradations inservice previously mentioned occurring at more exaggerated levels,compared to the channels available in each system results in pooreroverall service. It is a problem to promote competition in cellular PCSservice without degrading the level of service that could be provided bythe total channels available.

SUMMARY OF THE INVENTION

One of the objects of Applicants' invention is to provide a given HHTlocated in the system's service area with a multitude of broadcasttransceivers that the system may use for the communication between anHHT and the land line side of the cellular system or between individualcells or between other HHTs in the systems This may be readilyvisualized by having repeat copies of the existing prior art cellulararrangement which are set up so as to overlay the cells in the newlayers with the layers shifted geographically from each other. From eachpoint in the service region covered by the entire cellular system, eachpoint will be removed from a cell boundary in at least one layer (i.e.being closer to the center of a cell in at least one of the layers) Thiscan be seen graphically in FIGS. 2c and 2d, in which the seven-cellpattern of FIG. 2a is repeated by a three-layer replication where themidpoints of the cells in layers 2 and 3 are placed at the corners ofthe hexagonal cells which form the system shown in FIG. 1 (also FIG.2a). It should be noted that this invention is being illustrated usingthree layers; however, the system can be constructed with any number oflayers being used, so long as two or more layers are employed for agiven region (FIG. 2b).

As can be seen from FIGS. 2c and 2d, what occurs in a three-layer systemis a "triangular grid," in which any HHT in a given triangle is able toreceive service from transceivers at any of the corners of the triangle.In this type of arrangement, each transceiver of the three-layer systemwill generally have one-third the number of the frequencies that wouldbe allocated to a hexagonal cell in the single layer system. It shouldbe noted that current conventional systems might have multiplefrequencies for each cell.

As can be seen from FIG. 3, three transceivers for a preferredembodiment of our three-layer system (labeled transceiver levels a, band c, respectively) can service an HHT under the control of the CellSite Controller (CSC). This CSC may control other transceivers in thelocal region on various levels. The three levels a, b and c correspondto the "corners" of a triangle such as is shown in FIGS. 2c and 2d. TheCSC can perform the frequency assignments from an appropriate level a,b, or c transceiver and determine some hand-offs in this embodiment.

A new design is shown in FIG. 4 in which the Base Station Controller(BSC) is attached by voice and data links to a number of CSCs, each ofwhich is connected to a number of transceivers, each of which canconnect to a number of mobile units (HHTs) in its predetermined portionof region coverage. An HHT can be a hand-held telephone or other mobileunit communicating voice or data over an assigned frequency channel to aselected transceiver.

The CSCs and BSCs have the computing power to process the signalstrength data from one or more transceivers communicating with an HHTrequest for service and to determine which frequency channel assignmentthrough which transceiver will be initially allocated for communicatingwith the HHT, as well as any hand-off reassignment of channel andtransceiver necessitated by the HHT moving beyond the cell of thecurrently assigned transceiver.

As noted above, each of the layers could be serviced by a differentservice provider, presuming that some standard degree of cooperationexisted between the providers, such as is shown in the alternativeembodiment of this invention in FIG. 4. With this type of arrangement,what is generally referred to as the "A" carrier in a region couldprovide service for one layer through the level 1 CSC shown in FIG. 4,and the "B" provider provides a second layer using the level 2 CSCcontroller in FIG. 4. A third layer could be shared by the twoproviders, and, if additional layers are used, the rights to service anyremaining layer(s) could be auctioned in the same manner as otherfrequency auctions have taken place, increasing revenue to thegovernment. In this alternative embodiment, the determination offrequency assignment and the employment of hand-offs is controlled atthe base station level in the hierarchy by some approved standardprotocol. The system of this embodiment of this invention could, ofcourse, be serviced by a single provider providing the service on alllayers as well.

The previous problem of traffic surge that is discussed above isameliorated in that a surge in traffic which might occur over aparticular cell of FIG. 1 may be handled by only one antenna in theprior art system of FIG. 1. For example, in a system having thethree-layer arrangement covering the area shown in FIG. 2c, sevendifferent transceivers would be available to help with calls that wouldformally have existed in the area of a single hexagonal cell allowingapproximately 2.33 times as many calls as would previously have beenpossible. Computer simulations have shown that the percentage of thetotal system frequency capacity used by this invention with little or noblockage is significantly higher in this system than in the prior artsingle-layer system. Call blocking is the situation where no service canbe provided directly or with a hand-off for a new request in a cell.

In FIG. 5, an example of a road passing through a three-layer system isshown in which the improvement over the conventional arrangement isobvious. The capacity along the road shown is at least 1.54 times thecapacity of a conventional system. For example, using two frequencychannels per transceiver in the triangular grid corresponding to six pertransceiver in the hexagonal system, it is noted that the thirty-sevenhighlighted transceivers in the triangular grid serve the road withseventy-four channels maximum capacity, contrasted with only eighttransceivers of the hexagonal grid providing forty-eight channelsmaximum capacity to the road, providing the 1.54 capacity increase. Theburst area providing a clear 2.33 times improvement over a conventionalsingle-cell site in terms of capacity is also shown in FIG. 5.

FIG. 6a shows forty-three points requesting service for individual HHTsover an eight hexagonal-cell region where each cell has a capacity ofsix channel assignments. Note that two cells are at capacity wherefurthermore the eleven request points in one of these cells resulted infive blocked calls, and the eight points in the other capacitated cellresulted in two blocked calls. Although the total requests were only43/48=89.6% of total channel capacity, only 36 requests representing 75%of the capacity were handled by this conventional cellular system. Ingenerals, any surge in traffic in a given area of current art cellularsystems may result in blocked calls (i.e. no frequency pair channel isavailable), even though the average utilization of the network might besignificantly less than half or smaller than the network's peakcapacity. This is especially true in an urban area where a given numberof calls may originate from some specific area at some specific timeconsiderably more frequently than the average number of calls over thearea.

As can be seen from FIGS. 6a and 6b, the probability of potentialblockage of a call or the prevention of a call from continuing as an HHTmoves through the system is determined by the portion of the cells whichare saturated (the shaded areas in the figure) by having all possibletransceiver frequencies in use. A saturated cell is a cell in which alltransceivers have all of their frequency capacity in use. In aconventional hexagonal systems, blockage and saturation essentiallyoccur at the same time. In this invention, on the contrary, saturationdoes not imply blockage of an area. In facts, blockage occurs only aftera substantially greater number of service requests have been receivedand frequencies allocated employing this invention's hand-off strategy.Using FIGS. 6a and 6b as a comparison, the shaded area in a conventionalsystem that is blocked is greater than the very small shaded area of thetriangular grid shown in FIG. 6b. In FIG. 6a, 25% of the coverage areawould be blocked by two saturated cells. In FIG. 6b, about 4% ispotentially blocked, even after having serviced all forty-three requestscorresponding to FIG. 6a. Each of the dots in both FIGS. 6a and 6brepresent HHT 100 users. In FIG. 6b, using the three-layer approach, sixof the fourteen transceiver sites not on the boundary will still have atotal of six available channels for the coverage area. The twenty-twotransceiver sites on the boundary would be servicing twenty-one calls inthe region, allowing an additional twenty-three frequency channels intotal to be available for service internal or external to the regionshown. Only the 4% saturated area shaded would be prevented fromaccepting a new call by virtue of a lack of a channel for immediateassignment. As will be discussed below, even this problem of the 4%saturated area situation can be ameliorated to avoid blockage by thehand-off mechanism of this invention

Assignment in the triangular grid of a preferred three-level multilayerarrangement system is an advantage of this invention with respect toimproved utilization, and, therefore, improved coverage even withoutusing the improved hand-off feature of this invention, which, in and ofitself, is a useful and non-obvious improvement over prior artarrangements.

By having this multilayer arrangement, various mechanisms are used forallocating new calls in a given cell to the available frequency. Thisallocation from alternative transceivers of different levels may beaccomplished by either a strongest signal, a load balancing or aproportional availability strategy. All of these strategies provide fora fine-tuning of the network so as to minimize the number of hand-offsof moving HHTs across cell boundaries while still maintaining areasonable level of available new service in most regions and allowingfor the greatest expansion for "surge" type of problems. Theseallocation methods are discussed below in the detailed description ofthis invention and are a significant feature and object of thisinvention.

Furthermore, by using the hand-off structure of this invention, a "spacediversity channel reassignment" mechanism may be employed for providingservice to a new call in a triangular cell where all the transceivers atthe three corners of the triangle currently have all frequencies in use.This hand-off embodiment is also discussed below and may be used inconnection with the allocation mechanism embodiment discussed below.

Furthermore, this invention improves the finding of a "hand-off path,"where the sequence of hand-offs results in an available frequency andthe remote cell being utilized in the most efficient manner, and thefrequency that was previously used becoming available within the currentcell to be allocated to a new call. This lessens the chance of a blockedcall and increases the average utilization of the entire system.

Prior art systems such as Ito, S, "Design for Portable Telephone Methodsfor Enable Initiating and Receiving Calls from a Vehicle", IwatsuElectric Co., Ltd., Tokyo, Japan VTC 1989, Pages 136-141, which use atwo-layer system, suffers from problems in that it is primarily designedfor handling only one-dimensional fast-moving traffic and does not havethe benefits of frequency allocation and reassignment strategies, as isthe case in Applicants' invention as will be discussed below.

Systems such as are discussed in "Cellular System Design: An EmergingEngineering Discipline", February, 1986, Vol. 24, No. 2, I.E.E.E.Communications Magazine, employs a second layer of service using thesame antenna where the second layer of service covers only a part of theentire region. This provides an uneven level of service and, of course,does not use the frequency allocation and reassignment system that thisinvention provides.

This invention solves several known problems in the prior art systems.Specifically, the need for a frequency assignment strategy to providegreater utilization and effective capacity without the need forreduction in cell size.

The second known problem solved is the need to avoid any substantialoccurrence of call cut-offs during operating periods when the system issubstantially below full capacity.

This invention further solves the problem of assignment anomalies atcell boundaries.

This invention further solves the problems that occur in prior artsystems due to occasional equipment failures and random spikes in usagethat in prior art systems caused local cell blockage during periods ofonly moderate overall utilization.

This invention still further solves the problem of statisticaldegradation of total service by uncoordinated independent serviceproviders each using a portion of the frequency spectrum, by providing amechanism by which minimal coordinating standards can be set forcompeting providers to effectively statistically enhance overall servicecapacity.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 is a graphical description of the seven-cell repeat arrangementof cells in a conventional cellular network;

FIG. 2a is a graphical representation arrangement of a conventionalsystem;

FIG. 2b is a graphical representation of a corresponding two-layersystem;

FIGS. 2c and 2d are graphical representations of a three-layer systemaccording to this invention and a three-layer embodiment of thisinvention, in which the triangular cellular nature of the distinguishedoverlapped coverage areas are shown;

FIG. 3 is a system-wide implementation diagram of one embodiment of thisinvention;

FIG. 4 is a system-wide diagram of an alternate embodiment of thisinvention allowing for multiple service providers;

FIG. 5 is a diagram showing the capacity increase of this invention overthe prior art along a road and in a "burst" area;

FIGS. 6a and 6b are a graphical representation showing the improvementbetween a conventional hexagonal cell system and the capacity of athree-layer triangular cell implementation of this invention;

FIG. 7 graphically shows a frequency substitution methodology used toextend the use of a hand-off algorithm to blocked areas in thisinvention;

FIG. 8a graphically shows for a three-layer setup, the allocation methodusing a strongest signal scheme according to this invention;

FIG. 8b graphically shows for a three-layer setup, the allocation methodusing a load balancing signal scheme according to this invention;

FIGS. 8c and 8d graphically show for a three-layer setup, the allocationmethod using a proportional availability scheme according to thisinvention;

FIGS. 9a, 9b, 9c, 10a, 10b, 11a, 11b, 12a, 12b, 12c, 12d, 13, 14 and 15are graphical representations necessary for understanding the hand-offmechanism;

FIGS. 16a and 16b are a flowchart showing a hand-off mechanism accordingto this invention.

DETAILED DESCRIPTION OF THE INVENTION

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is,therefore, to be understood that within the scope of the appendedclaims, the invention may be practiced otherwise than as specificallydescribed herein.

As an example, a three-layer system is used as an illustration in agiven geographic area serviced by three transceivers (one from eachlayer in a three-layer system), as shown in FIGS. 8a-8d, where thenumbered dots are HHT's that are requesting a connection to the systemin numerical calling order. In FIG. 8a, where the connection will go tois based, for example, on the strongest signal received by the HHT(generally the closest geographically) e as noted in FIG. 8a. Currentstate-of the-art HHTs without additional equipment can provide therequired information to perform this allocation, and a full descriptionis omitted as one of ordinary skill in the art would readily understandthe concepts involved. This is believed to be a good initial allocationof transceiver to HHT for fast-moving traffic.

In FIG. 8b, which would be the best for slow-moving HHTS, a balancing ofload between the three transceivers is used. This causes the number ofHHTs per transceiver to be as close to the same as possible over a broadregion. This allocation method serves to best spread the availablefrequencies over transceiver sites, thereby deferring the need forhand-offs, especially for slow-moving HHT traffic. This approach in amultiple service provider system may be a more useful allocation methodthan the allocation method of FIG. 8a.

In FIGS. 8c and 8d, a method which the inventors call proportionalavailability is used for the allocation. In the simplest form, a simplefunction using signal strength and load balancing is used to provide forimprovement in future requests for service (i.e. HHT No. 7 requestingservice in FIG. 8d) and for the possibility of better handling ofearlier boundary crossing. FIG. 8d further shows the possibilities ofthe different allocation strategies assigning HHT 7 to differenttransceivers.

Alternatively, as noted with respect to FIG. 4, a hybrid approach to theallocation can be employed which can tare into account, for examples,multiple service providers or other reasons for which a preference otherthan signal strength or load balancing is taken into account. Theallocation of available channels in the multi-layered system of thisinvention will now be described in reference to the flow chart of FIGS.16a and 16b. As can be seen, as an HHT user initiates a request to placea call (i.e. a request for service), the given HHT searches for thestrongest radio setup channel, identifying a channel from themulti-levels. For the purposes of this discussion, call this level A,through a transceiver of level a. A request for service message willthen travel over a link to the CSC and then to the BSC. The CSC or theBSC, if necessary, then directs those transceivers from the other levels(B and C in this case) whose range overlaps the particular transceiverfrom the A level to monitor the signal strength of the HHT to determinethe closest level B and level C transceivers.

The CSC then assigns one of the three transceivers from levels a, b or cto service the communication request based on an algorithm that willfactor in the relative strength of the signals and the availablefrequencies at each of the three transceivers. This allows for using thestrongest signal, load balancing and proportional allocation.

The communication then proceeds through the conventional process ofauthentification, digit collection, analysis, validation and other callset-up functions. The link between an HHT and a transceiver is over theair using any of the known RF link methodologies. The links from atransceiver to a CSC and CSC to BSC are preferably made by wire and/ormicrowave, but can also be by fiber optic or other means.

If the user moves out of the cellular area covered by the assignedtransceiver or the system needs to reallocate the transceiver, ahand-off employing a methodology, which will be set forth below, or evena conventional hand-off, can be used to maintain the communicationlinks. The call will only be terminated by the system if no newconnection can be founds; however, as will be set forth below, theprobability of this occurring using the hand-off mechanism of thisinvention decreases over prior art systems dramatically.

In the arrangement shown using a "three-layer embodiment" shown in FIG.2, various hand-offs between individual elements within the cell canoccur by various mechanisms and procedures. By having the multipletransceivers in the layered arrangement, the hand-off system can beoptimized, and the usage of the system when there is a passage of an HHTthrough the system can be improved. This arrangement and hand-offmethodologies allow a more uniform usage of the freguencies involved inthe entire system and additionally allows a system in which multipleservice providers may use or share the frequency bandwidth moreadvantageously. Further, some or most of the techniques that have beenused to increase conventional systems capacity can also be used toprovide further increases in this invention's capabilities.

In this invention, as shown in FIGS. 2c and 2d, a greater level ofservice can be provided. Specifically, in FIG. 9a, if X represents acell transceiver and Y represents a hand-held unit in the cell, and if Xcovers Y (i.e. Y is located within X's service range), a solid line willconnect X and Y as shown in FIG. 9a. If Y has been served by X (i.e. achannel is assigned to Y from the tranceiver X), a broken line willconnect X and Y. A broken and solid line indicates a covered, as well asan assigned, frequency and a solid line indicates covered but not yetassigned frequency.

For example, as shown in FIG. 9b, if the hand-held transceivers Y1 andY2 are in the service area of cell X1, but Y3 is not in the servicearea, Y2 is being serviced by X1. To illustrate the hand-off chainconcept of this invention, an alternating path would be shown as in FIG.9c, with the edges alternating between covered but not assigned andcovered and assigned frequencies. In the example shown in FIGS. 10a and10b, a single hand-off of the hand-held unit Y is shown between the cellsites X1 and X2 where there is an overlap between X1 and X2. A doublehand-off of HHTs is shown in FIGS. 11a and 11b, using, for example, thecell sites X1, X2 and X3. By having extensive overlap of the cells inthe triangular grid arrangement shown in FIG. 6b, the passage of a givenHHT through the system can be more easily accomplished. Note in FIG. 6bthat a single hand-off would free a channel in either of the saturatedtriangular cells (shaded). Furthermore, a chain of hand-offs, as shownin FIGS. 12a, 12b, 12c and 12d, can likewise be achieved, traffic inindividual cells, thereby using the frequency bandwidth much moreefficiently.

As shown in FIG. 13, using the cell sites X1, X2 and X3 as an example,the hand-held transceiver Y1, which is generally in the service area ofX1, may be unable in a conventional system from achieving a connectionbecause a given cell X1 would or could be saturated or, for that matter,out of service or could not provide service for some reason. However, asshown in FIGS. 14 and 15, transceiver X1 can provide service for a "new"HHT in this area in this invention.

In the above example of FIG. 13, even if X2 is also saturated but X3 isnot, it is possible to find an alternating path for the HHT Y1 which isterminated at X3 (a transceiver station with a free frequency) by doingthe chain of hand-offs to serve Y1 by using the alternating path shownin FIG. 14. By first handing off Y3 to X3, then Y2 to X2, a free channelis then made available in X1 to serve Y1. Afterwards, a hand-offsequence could occur as shown in FIG. 15. This allows for an HHT toenter a saturated cell or to originate a phone call in a saturated cell,preventing the blocking problem that occurs in prior art systems.

What occurs in this invention is a solution to the blocking problem thatoccurs in conventional single-layer cell arrangements, and in which afeature of Applicants' invention is that an alternating path that startsfrom a given HHT and ends at a transceiver with free frequencies isemployed. This uses the augmenting path graph theory and which followsthe flow diagram shown in FIGS. 16a and 16b. Using conventional graphtheory terminology to assign a frequency from a base station to ahand-held transceiver would be equivalent to the matching that occurs inconventional graph theory if the transceiver has K frequencies to beassigned. This will be a K-matching problem in bipartite graphs.Starting from Y to find the augmenting path that ends at X, with thebreadth first search being guaranteed to find the shortest path (forexample, the least number of hand-offs required), and which is linear intime (i.e. the number of steps to find the path is proportional to thesize of the path found plus the number of HHTs searched). The followingare definitions:

Q--FIFO Queue (first in, first out queue)

K--Let K be the number of channels allocated to cell tranceiver X.

Empty Qx--initialize Qx to be an empty queue

Empty Qy--initialize Qy to be an empty queue

Mark x--initialize mark flag for x

Mark y initialize mark flag for y

Qx←x--enter x into bottom of Qx

Qy←y--enter y into bottom of Qy

This follows the flowcharts as set forth in FIGS. 16a and 16b.

The hand-off chain algorithm will not work when an HHT user crosses aboundary within a blocked region (i.e. a region where all frequencies inevery transceiver are busy, and no hand-off chain to an availablefrequency is possible), unless frequency substitution is made asfollows: When the algorithm fails to find a hand-off chain for the HHT'snew call, the previous frequency is released as an available frequencyand the hand-off chain search is re-initiated.

If a hand-off chain is then found as shown in FIG. 7, the tail end ofthis chain will be the frequency just added. (This chain is a loop,since the head and tail of the chain are the same.) The above extensionof the hand-off algorithm by frequency substitution will allow an HHT tomove from cell to cell in a totally blocked region.

But there is a drawback--the hand-off operation will cause the HHT tosuffer a transitory communication loss. The current technology requiresa bridging period and switching period during the hand-off operationbetween the "hand-off from" and "hand-off to" frequencies to betransparent. These consist of:

a) Bridging (carrying the call on both frequencies); and

b) Actual switching between the two frequencies. In an unblocked area(having a frequency available for bridging), loss of communication onlyoccurs during step b (e.g. approx. 100 microseconds). In a blocked area,the HHT frequency must be surrendered for bridging, so communicationloss occurs during both steps a and b.

To implement this frequency substitution to allow an HHT user to moveacross a cell boundary in a blocked-region, the longer the hand-offchain, the greater the communication loss period. But this drawback onlyapplies to the HHT at the head of the hand-off chain, i.e. the one whosemovement necessitated the hand-offs. Also, a limit can be imposed on useof the substitution (i.e. such a substitution will be allowed only whenthe hand-off chain is short enough to ensure that the loss ofcommunication will not exceed a preset acceptable loss duration).

What is claimed as new and is desired to be secured by Letters Patent ofthe United States is:
 1. A load balancer for use in a cellularcommunication system for communicating with mobile phones and includinga plurality of base stations, for communicating with said mobile phones,each of said base stations coupled to a plurality of transceivers, eachof said plurality of transceivers defining a cell in which thetransceiver selectively provides frequency channel assignments to saidmobile phones within said cell, the cells being arranged in a cellularpattern, said transceivers covering a geographic service area and beingpartitioned such that respective cells form plural cellular layers ofcommunication, each of the plural cellular layers substantially coveringall of said geographic service area; substantially all of the cells ofeach of the plural cellular layers overlapping a plurality of cells ineach other cellular layer such that at least first and secondtransceivers from different cellular layers of the plural cellularlayers selectively provide a frequency channel assignment to arequesting mobile phone requesting a frequency channel assignment withinthe geographic service area, the load balancer comprising:a selector forselecting between at least first and second transceivers from thedifferent cellular layers based on load balancing between said at leastfirst and second transceivers, such that a selected one of said at leastfirst and second transceivers and a corresponding base station of saidplurality of base stations provide said frequency channel assignment forsaid requesting mobile phone.
 2. A system as in claim 1, wherein saidselector includes means for obtaining said frequency channel assignmentfor said requesting mobile phone by causing the selected transceiver tohand-off another mobile phone that is currently assigned said frequencychannel assignment to another frequency channel assignment provided by athird transceiver defining a respective cell in one of the pluralcellular layers, said requesting mobile phone being outside the cell ofsaid third transceiver.
 3. A method of assigning a frequency channel toa requesting mobile phone requesting a frequency channel assignment in acellular communication system that includes a plurality of basestations, each base station coupled to a plurality of transceivers, eachtransceiver defining a respective cell, the cellular communicationsystem including first and second transceivers defining respective firstand second cells, each of the first and second transceivers selectivelyproviding frequency channel assignments to requesting mobile phoneswithin the respective first and second cells, the first and second cellsbeing arranged in an overlapping cellular pattern covering a geographicservice area, said overlapping cellular pattern being arranged such thateach cell overlaps a plurality of other cells, thereby defining aplurality of respective overlap regions such that, within a givenoverlap region, a requesting mobile phone is able to receive a frequencychannel assignment from a transceiver for any cell covering such overlapregion, the method comprising the steps of:assigning to a firstrequesting mobile phone a first frequency channel assignment from saidfirst transceiver, said first requesting mobile phone being within anoverlap region defined by an overlap of the first cell defined by saidfirst transceiver and the second cell defined by said secondtransceiver; assigning to a second requesting mobile phone a secondfrequency channel assignment from said second transceiver, said secondrequesting mobile phone being within said overlap region; receiving atsaid first transceiver a handoff request for a new frequency channelassignment from said second requesting mobile phone that is within thefirst and second cells but is moving out of the second cell; andresponsive to receiving the handoff request from said second requestingmobile phone, the method further comprising the sub-steps of: releasingsaid second frequency channel assignment; handing-off said firstrequesting mobile phone to said second frequency channel assignment fromsaid second transceiver; releasing said first frequency channelassignment; and reassigning said first frequency channel assignment tosaid second requesting mobile phone.